Thermal convection flow light valve

ABSTRACT

A light valve includes a cell containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid, and means for applying such a field thereto. Specifically described is a fluid suspension of minute particles acted upon by an electric field. An external or internal conduit connects opposite edge portions of the cell and heat is applied to the fluid in the cell or conduit to produce a thermal convection flow of the fluid through the cell and conduit.

. United States 11 3,788,729 Lowell et al. Jan. 29, 1974 1 THERMAL CONVECTION FLOW LIGHT 2,290,581 7/1942 061161, Jr. .0 350/267 VALVE 2,481,621 9/1949 Rosenthal... 350/267 3,292,171 12/1966 Wilson 350/267 [751 In rs: a s Lowell, yn; Matthew 3,507,551 4/1970 Stetten 350/160 Forline, Ozone Park; Paul Rosenber L ch t; R bert L. saxe s g gifg Primary Examiner--R0nald L. W1bert Assistant Examiner-V. P. McGraw 1 Asslgneei fi y Frontiers, Plamvlew, Attorney, Agent, or Firm-Stephen E. Feldman 22 Filed: Apr. 28,1972

[57] ABSTRACT [21] Appl. No.: 248,479

A light valve includes a cell containing a fluid capable Related Application Data of being acted upon by an electric or magnetic field,

[63] Continuation of March 1971' or both, to change the transmission of light through abandoned the fluid, and means for applying such a field thereto. Specifically described is a fluid suspension of minute g 'i 350/160 g g; particles acted upon by an electric field. An external or internal conduit connects opposite edge portions of [58] Field-0f Search 350/147 267 the cell and heat is applied to the fluid in the cell or [56] References Cited conduit to produce a thermal convection flow of the UN TED STATES PATENTS fluid through the cell and conduit.

1,963,496 6/1934 Land 350/267 24 Claims, 17 Drawing Figures PATENTEDJAIZQIFJH 3,788,729

SHEET 1 [IF 4 I9 INVENTORS 1s FRANCIS c. LOWELL 3 MATTHEW FORLINI PAUL ROSENBERG ROBERT L. SAXE ATTORNEYS PATENTEDJAN 29 m4 SHEEI 2 OF 4 FIG LI E E 3 I Y TO N N F R EQ ST 0 ws mm 1% w A NTL AM R A FM PATENTEDJANZQ 1914 3,788,729

SHEU 3 OF 4 FIG. 7

FIG. 9

65 Well I Ourslde I Ver'ricol Conduit H of Room Temperature [NVENTORS ATTC) RNEY S PATENTEDJAMZQlQM 3,788,729

SHEET LL 0? 4 I! w 74%" 73 v 775 270 75 v 7s I I 3x3} 12 Area Elecrrode s ji 12 7| I r 72 Area Electrodes FIG. 16

+ INVENTORS FRANCES c. LowELL MATHEW FORLINI PAUL ROSENBERG ROBERT L. SAXE BY Q ATTORNEYS THERMAL CONVECTION FLOW LIGHT VALVE This is a continuation of application Ser. No. l29,873, filed Mar. 3l, l97l, and now abandoned.

BACKGROUND OF THE INVENTION This invention relates to light valves of the type including a cell containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through fluid.

The invention is particularly applicable to light valves utilizing a fluid suspension of minute particles dispersed therein. Light valves of this type have been known for many years. Fluid suspensions of herapathite in a suitable liquid have commonly been preferred, although other types of particles have been suggested. In general, the shape of the particles is such that in one relative arrangement they intercept more light than in another relative arrangement. Particles which are needle-shaped, rod-shaped, lath-shaped or in the form of thin flakes have been suggested. The particles may variously be light absorbing or light reflecting, polarizing, birefringent, metallic or non-metallic, etc. In addition to herapathite, many other materials have been suggested such as graphite, mica, garnet red, aluminum, periodides of alkaloid sulphate salts, etc. Preferably dichroic, birefringent or polarizing crystals are employed.

Very finely-divided or minute particles are employed, and are suspended in a liquid in which the particles are not soluble, and which is of suitable viscosity. In order to help stabilize the suspension when in the non-actuated state, a protective colloid should preferably be used to prevent agglomeration or settling.

A fluid suspension which has been used with success uses generally needle-shaped particles of herapathite, isopentyl acetate as the liquid suspending medium, and nitrocellulose as a protective colloid. Plasticizing agents such as dibutyl phthalate have also been used in the suspension to increase the viscosity.

Both electric and magnetic fields have been suggested for aligning the particles, although electric fields are more common. To apply an electric field, conduc tive area electrodes are provided on a pair of oppositely disposed walls of the cell, and an electric potential applied thereto. The electrodes may be thin transparent conductive coatings on the inner sides of the front and rear walls of the cell, thereby forming an ohmic type cell wherein the electrodes are in contact with the fluid suspension. It has also been suggested to cover the electrodes with a thin layer of transparent material such as glass in order to protect the electrodes. Such thin layers of glass form dielectric layers between the electrodes and the fluid suspension, and the cells may be termed capacitive cells. Direct, alternating and pulsed voltages have been applied to the electrodes in order to align the particles in the fluid suspension. When the voltage is removed, the particles return to a disoriented random condition due to Brownian movement.

Commonly the front and rear walls of the cell are transparent, for example, panels of glass or plastic. With no applied field, and random orientation of the particles. the cell has a low transmission to light and accordingly is in its closed condition. When a field is applied, the particles become aligned and the cell is in its open or light transmitting condition. Instead of making the rear wall transparent, it may be made reflective. In

such case light is absorbed when the cell is unenergized and is reflected when the cell is energized. These principal actions may be modified by employing light re 'flecting rather than light absorbing particles. It is also possible to select types of particles and applied fields such that application of a field closes the cell and removal of the field opens the cell.

In such cells a serious problem is the agglomeration of the particles. While protective colloids are helpful in reducing or avoiding agglomeration in the stored or inactive condition, when the cell is in use the tendency to agglomerate increases. Depending on the particular suspension employed, and the voltage and frequency used, agglomeration may become noticeable in a matter of seconds, minutes or hours of use. Once agglomeration has occurred, it tends to remain more or less permanently even though the exciting voltage is removed.

Such agglomeration considerably impairs the usefulness of the light valve since it creates inhomogeneities in the suspension and hence changes the light transmission from point to point. Also it reduces the ratio of optical density between the closed state and the open state. Further, the density in the closed state may decrease.

The possible setting of particles out of the fluid suspension has previously been recognized. In this connection it has been suggested to furnish a constantly renewed supply of particles to the suspension, or to stir up and redistribute settled particles, or to cause a slight current or flow of particles within the container so as to assure a constant suspension. While such expedients may help to avoid settling, agglomeration may still occur during use. Also, the turbulence involved may appreciably affect the performance of the cell.

In copending application Ser. No. 25,542, filed Apr. 1, 1970 by Matthew Forlini for Light Valves with High Frequency Excitation," the use of frequencies higher than heretofore proposed is described, in order to avoid agglomeration of the particles in the suspension. While effective, the use of high frequencies involves considerable power consumption in the high frequency power source, and the power source may be quite expensive.

In copending application Ser. No. 174,494 filed Aug. 24, l97l and now US. Pat. No. 3,708,2l9 by Matthew Forlini et al. for Light Valve with Flowing Fluid Suspension" a circulating system is described for producing a flow of fluid suspension through the cell during operation thereof to reduce or avoid agglomeration of the particles. Various means are described for producing a smooth generally laminar flow of the fluid suspension in the active region of the cell. Mechanical pumps are particularly described. While effective, the pumps may be relatively costly, bulky, heavy and somewhat noisy, and may produce undesirable mechanical vibrations in the cell. Also substantial power may be required to drive the pump.

The present invention is directed to producing a flow of the fluid suspension by less expensive, less bulky and lighter means which is completely free of vibration and noise and requires a relatively small amount of power for operation.

SUMMARY OF THE INVENTION In accordance with the present invention, thermal convection flow of the fluid or fluid suspension through the cell is produced. To this end, fluid conduit means connects substantially opposite edge portions of the cell and the conduit means and cell are arranged so that the conduit means or cell, or both, have a portion thereof extending in a vertical direction, or in a direction having a substantial vertical component sufficient to yield the desired thermal convection flow. Means are provided for producing thermal gradient in the fluid in the portion having a substantial vertical component to produce a thermal convection flow of the fluid through the cell.

Preferably the spacings of the walls of the cell at the opposite edge portions connected by the conduit means are substantially greater than the spacing of the wall sections in the active region of the cell so as to form channels promoting a smooth generally laminar flow in the active region.

In certain embodiments of the invention both the cell and the conduit means extend vertically, or in a direction having a substantial vertical component, and the conduit means connects channels at the upper and lower edges of the active cell region. A differential temperature is produced between at least a portion of the fluid suspension in the cell and at least a portion of the fluid suspension in the conduit means, thereby producing a thermal gradient in the vertical direction which causes a thermal convection flow of the fluid suspension through the cell. The conduit means may be one or more conduits positioned exteriorly of the cell, or may be built integrally with the cell structure so as to extend upwardly on one or both sides of the active cell region and separated therefrom by barrier walls. The differential temperature may be produced by applying heat to the conduit means. Or, heat may be applied to a laterally extending region of the cell, preferably below the active region thereof.

In other embodiments of the invention, the cell, conduit means and heating are arranged so that fluid flow in the active area of thecell is generally horizontal, although still produced by thermal convection. This is advantageous in the case of long narrow vertical cell panels where the long dimension is horizontal, and also permits the cell panels to be oriented in a horizontal plane.

Broadly, in these other embodiments the cell and at least a portion of the conduit means are located at different vertical levels, and heat is applied at a suitable region to produce a thermal convection .flow. As specifically illustrated, the channels connected by the conduit means are at opposite horizontally-separated edges of the active cell region, and heat is applied to an upwardly-extending portion of the conduit means, or in a channel connected to such an upwardly-extending portion.

Electric heating coils, resistive coatings, heating wires or ribbons, etc. may be employed, energized with A-C or D-C from a suitable power source. Other sources of heat (or cold) may also be employed, if desired, as will be mentioned hereinafter. Control of the heating is desirable in order to permit obtaining the maximum rate of flow without an excessive temperature rise which might degrade the suspension.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 2 is a face view of the arrangement of FIG. I, with portions broken away;

FIG. 3 is a cross-section taken along the line 3-3 of FIG. 2;

FIG. 4 is a face view of another embodiment of the invention, with portions broken away, using an internal conduit with a heating strip along the bottom of the cell;

FIG. 5 is a cross-section taken along the line 5-5 of FIG. 4;

FIG. 6 shows another embodiment of the invention, with portions broken away, using internal conduits with heating means located therein;

FIG. 7 is a horizontal cross-section taken along line 7-7 of FIG. 6;

FIG. 8 shows an alternative structure to that shown in FIG. 7;

FIG. 9 illustrates a light valve with external conduit mounted in the wall of a building;

FIG. 10 is a cross-section taken along the line 10-10 of FIG. 9;

FIG. 11 illustrates an embodiment of the invention in which fluid flow in the cell is horizontal rather than vertical, and FIG. 12 is a cross-sectional along the line 12-12 of FIG. 11; and

FIGS. 13-17 illustrate several different conduit and cell arrangements for horizontal fluid flow.

DESCRIPTION OF THE SPECIFIC EMBODIMENTS Referring to FIGS. l-3, a light valve generally indicated as 10 is formed of two sheets 11 and 12 of transparent material such as glass, plastic, etc. Transparent conductive coatings 13 and 14 are formed on the inner surfaces of sheets 11 and 12 to form parallel area electrodes in the desired active area of the valve. As shown, the electrodes are in contact with the fluid suspension, thereby forming an ohmic type cell. If desired they could be covered with a thin layer of transparent material such as glass to form a capacitive type cell.

In the region between coatings 13 and 14, the front and rear walls are spaced apart a distance which is small compared to the lateral dimensions of the wall sections so as to confine the fluid suspension therebetween to a layer. Spacings of the order of ID to 25 mils are advantageously employed, although spacings outside this range may be used if desired.

The spacings between the front and rear walls at the upper and lower portions of the cell are substantially greater than the spacing of the wall sections in the region of the layer, as shown at 15 and 16 in FIG. 3, and may be of the order of one-half inch, for example. These relative dimensions are impractical to show in the drawings. The transparent sheets are separated by thin spacers on each side of the cell, one of which is shown at 17 in FIG. 1, and by thicker spacers at the top and bottom, shown at 18 and 19. The spacers may be of sealing material or cemented to the transparent sheets, etc. to produce a completely sealed fluid-tight enclosure for the fluid suspension. The upper and lower regions 15 and 16 of the cell are connected by a conduit 21. Spacers and sealing materials are desirably inert to the suspension. For example, spacers such as glass, or a plastic such as polyethylene, polypropylene or epoxy-filled fiberglass, may be employed.

The entire cell and conduit are filled with a suitable fluid suspension of minute particles dispersed therein capable of orientation by an electric field applied between area electrodes 13 and 14. To avoid confusion, the fluid suspension is not specifically shown in the drawings, but arrows indicate the thermal convection flow thereof as will be described below.

Heating means are provided for heating the fluid suspension in the conduit 21 and here takes the form of an electric heating coil 22 enclosed in the heating insulating casing 23. Advantageously the turns of the coil are closely spaced to avoid local hot and cool sections. The heating coil may be energized in any suitable manner, either with A-C or D-C, as desired. In FIG. 1, a plug 24 is adapted to be connected to the power mains and a switch 25 and variable resistor 26 are inserted in series with the coil to control the energization thereof.

When heat is applied by coil 22, the heating of the fluid suspension causes a thermal convection flow of the fluid suspension as indicated by the arrows in FIGS. 2 and 3. As the fluid suspension is heated by coil 22, it expands and its specific gravity decreases. Accordingly, the heated suspension rises in conduit 21, thereby forcing the suspension above it upward and into the chanml at the top of the cell and drawing the suspension from the channel 16 at the bottom of the cell into the lower portion of the conduit 21. This causes a downwardly flow of the suspension in the thin layer between electrodes 13,14. The suspension gradually cools as it circulates until it again reaches the heating region of coil 22, so that a differential temperature is maintained between the suspension in the heating zone of the conduit and that in the cell. Thus a continuous thermal convection flow is produced.

By making the upper and lower regions 15 and 16 of substantially greater spacing than that between electrodes 13,14, the resistance to fluid flow in 15 and 16 can be made much less than that between the electrodes. Thus the flow in the layer between the electrodes is smooth and substantially laminar, rather than turbulent, so that the flow does not interfere with proper operation of the light valve.

The area electrodes 13 and 14 are energized in any desired manner from a source generally indicated as 27, in order to vary the light transmission of the cell.

Inasmuch as the fluid suspension in regions 15,16 is not in the active region of light control, these regions may be made opaque by suitable coatings, framing, etc.

The heating coil 22 is shown approximately at the middle of conduit 21, which is considered preferable. However it can be positioned higher or lower if desired. By adjusting resistor 26, the heating can be controlled to yield a suitable convection flow without raising the temperature of the fluid suspension to a degree which will degrade the suspension. The conduit 21 is advantageously of metal having good heat conductivity to promote more efficient and rapid thermal convection flow, although other materials could be used if desired. For example, if heating means is provided inside the conduit, it may be advantageous to form the conduit of heat insulating material.

Referring to FIGS. 4 and 5, an internal conduit 31 is formed on one side of the cell by a barrier wall 32 of heat-insulating material. The barrier wall is sealed to the front and rear walls of the cell and is spaced from the corresponding side 33 of the cell. It is also spaced from the top and bottom of the cell to form a closed path for the circulation of the fluid suspension. The spacing of the walls at the upper and lower portions 15 and 16 of the cell is greater than that between the area electrodes, as in the previous embodiment, and the spacing at conduit 31 is also greater so as to reduce resistance to fluid flow therein.

In this embodiment the heating of the fluid suspension is along the bottom of the cell, rather than in the conduit. To this end laterally-extending electrical resistive coatings 34 and 35 are formed on the inner surfaces of the cell walls and spaced from the area electrodes 13 and 14 as indicated at 36 and 37 so as to be insulated therefrom. The resistive coatings 34,35 are supplied with heating current from a suitable source 39 under the control of switch 25 and rheostat 26. If desired, a resistive coating on only one cell wall could be employed. These resistive coatings need not be transparent, since they are below the active region of light transmission control.

Inasmuch as heat is applied below the active cell region in this embodiment, the fluid suspension will rise by thermal convection as indicated by arrows 41, and will return downward through conduit 31, as indicated by arrows 42. The suspension will gradually cool as it circulates until it again reaches the heating zone.

Instead of using a resistive coating on the inner walls of the cell, electrical strip heaters may be employed. If necessary, the heating coating or elements may be covered with a thin layer of insulating material such as a thin layer of glass so as to insulate the heating element from the fluid suspension and the electrodes 13, 14. Alternatively, a heating wire or wires may be embedded in the glass sheet or sheets, or wrapped around the bottom of the cell.

The embodiment of FIG. 4 with an internal conduit 31 is more compact than that of FIG. 1 and hence advantageous for many applications. If desired, however, an external conduit may be used as in FIG. 1, thereby promoting more rapid cooling of the suspension outside the heating region 34, 35.

It is possible that in some instances coatings 34, 35 could be made conductive and the power source connected between them to heat the lower region of the suspension by current flow therethrough. This is not preferred at the present time since the suspensions commonly employed are of high resistance even in thin layers and sufficient heating would be difficult to obtain.

The light transmission of the valve is controlled by source 27, as in the previous embodiment. The connections from source 27 to the area electrodes 13 and 14 are shown conventionally as wire leads 28, 29. In practice, conductive coatings may be formed on the inner surfaces of the front and rear sheets and brought out to the edge of the cell, so as not to interfere with fluid flow in conduit 31. Regions 15, 16 and those of coatings 34, 35 and conduit 31 may be made opaque if desired, since they are outside the region of light control.

Referring to FIGS. 6 and 7, internal conduits 51 and '52 are provided on each side of the cell by barrier walls 53 and 54, and heating wires 55 and 56 are mounted vertically therein by insulating plugs 57. The heating wires are energized in any suitable manner as indicated by power source 58. Suitable current control may be employed as described in connection with previous em bodiments. The fluid suspension will rise by convection in both conduits 51 and 52, and flow downwards between the area electrodes.

In this embodiment, instead of sealing the glass walls together around their peripheries, a U-shaped metal frame 61 is employed, e.g., of stainless steel. As indicated in FIG. 7, the barrier walls 53, 54 establish the desired separation of the glass walls between area electrodes 13 and 14. The metal frame 61 is then sealed to the exterior surfaces of the walls by a suitable adhesive. Insulated coatings extending vertically on the walls of conduits 51, 52 could be used in place of resistance wires 55, 56, if desired.

FIG. 8 shows an alternative arrangement in which one glass wall extends beyond the other and an L- shaped metal frame 62 is employed which is sealed to the glass walls.

A metal frame of'the type shown in FIGS. 6-8 could also be employed in the previous embodiments if desired, and in general would promote cooling of the suspension outside the heating region.

FIGS. 9 and 10 show an arrangement of the type of FIGS. 1-3, but with the heating coil 22 and insulating casing 23 omitted, mounted as a window in the wall of a building so that a differential temperature between cell and conduit is established without requiring a specific heat source. Here the cell is mounted in an opening in the wall 65 with the conduit 21 extending inside the building. As specifically shown, the conduit 21' is connected at the top and bottom edges of the cell rather than at the top and bottom of a side edge, but still serves to connect channels and 16. Connection to the side edge as in FIGS. 1-3 would be possible provided the adjacent room wall 65 is cut away so that the conduit 21' is still exposed to the inside temperature.

When the outside temperature is lower than the inside, the fluid suspension in conduit 21' will be at a higher temperature than that in the cell, and thermal convection flow will occur as desired in connection with FIGS. l3. When the outside temperature is higher, thermal convection flow will be in the opposite direction. If necessary, additional thermal insulation can be placed between conduit 21' and wall 65.

It is possible, of course, for the outside and inside temperatures to be the same. However, a temperature differential may still be present due to the difference in radiation impinging on the cell and conduit, from sunlight for example, and wind may disturb the thermal equilibrium. Although the temperature differential in FIG. 9 is less positive than in the other embodiments, and convection flow may sometimes be less strong, the embodiment has the advantages of simplicity and lower cost and requires no source of electric power.

The embodiments of FIGS. 4 and 6 could also be used in the wall arrangement of FIG. 9, and the specific heating means eliminated. In such case the internal conduits can be mounted in the wall so as to be insulated on the outside but exposed on the inside to room temperature.

It is possible also to arrange the conduits so as to be exposed to the outside temperature, since the cell walls will be thermally exposed to both outside and inside temperatures and hence the fluid suspension in the active cell region will be at an intermediate temperature.

In the preceding embodiments the cells are assumed to extend in a vertical direction, or at least in a direction having a substantial vertical component, and the flow of the fluid suspension in the active cell region is generally upward or downward. In subsequent embodiments, fluid flow in the cell is generally horizontal.

FIGS. 11 and 12 show the cell 10 disposed in a vertical plane, with channels 71 and 72 located at the side edges of the active cell region between area electrodes 13 and 14 so that the channels are horizontally separated. The cell construction is otherwise like that shown in FIGS. 1-3 and need not be described in detail. Energization of the electrodes is omitted for simplicity. Conduit has a horizontal portion 73 and vertical portions 74, 75 connecting channels 71 and 72. A heat source is arranged to heat the fluid suspension in one of the vertical portions 74, 75 here shown as an electrical heating coil 76. The coil would normally be heat insulated as illustrated in FIG. 2, but is shown only diagrammatically for simplicity of illustration.

With heat applied, the fluid suspension rises in conduit section 74 and flows through the conduit, as shown by arrow 77, to channel 72 and thence through the active cell region between electrodes 13,14 to channel 71 and back to conduit section 74. Since the layer between area electrodes 13, 14 is very thin compared to the corresponding dimensions of channels 72 and 71, the fluid flow across the cell is generally laminar.

In subsequent embodiments the active cell region between area electrodes 13, 14 is indicated by dash lines, and channels 71, 72 are denoted CH.

FIG. 13 is similar to FIG. 11 except that the cell 10 is located at a higher vertical level than the conduit 70, and channels CH are tapered to join with the conduit. The tapered configuration could be used in FIG. 11, or the ends of the conduit could be connected to the bottom of channels CH in FIG. 11.

In both the arrangements of FIGS. 11 and 13, the cell 10 could be disposed in a horizontal plane with conduit 70 above or below the cell, suitable bends being introduced in the conduit so that the portion thereof at which heat is applied extends vertically, or in a direction having a substantial vertical component. Fluid flow will still be in the direction indicated by the arrows.

FIG. 14 is similar to FIG. 11, but here a resistance heating element 78 is positioned in the outlet channel 71' to heat the fluid suspension. Advantageously a barrier wall 79 is positioned in channel 71 between the heating element 78 and the adjacent edge 81 of the thin layer of fluid suspension between the area electrodes, so as to avoid local circulation currents near edge 81. Barrier wall 79 extends from the upper toward the lower edge of the cell and is spaced from the lower edge to allow flow of the fluid suspension thereby as indicated by arrow 82.

FIG. 15 is similar to FIG. 11, except that the conduit is connected to the outlet channel at the bottom thereof, rather than the top. v

FIG. 16 shows the conduit connected to lateral edges of the channels CH, theconnection'to the inlet channel being near the bottom thereof and the connection to the outlet channel being near the top thereof.

FIG. 17 is similar to FIG. 13, but here cell 10 is disposed in a horizontal plane, or generally horizontally, thereby enabling it to be used in the roof of a vehicle, as a skylight, in a greenhouse, etc. where light control is desired. The conduit sections 74", 75" are bent laterally so that the connecting portion (not shown) will not block the beam of light through the cell.

The specific embodiments show rectangular cells which are likely to satisfy a wide variety of applications. However the cell shape may be different if desired, such as round, square, tapered, etc. Also, cells could be arranged in series or parallel with a single recirculating conduit and heat source. In the series arrangement one cell would form part of the conduit means for the other cell.

The specific heat sources in the embodiments of FIGS. 1-8 are electrical in nature and are generally convenient to use. However, radiant heat sources designed to concentrate heat in the desired regions could be employed if desired, or other suitable means for applying heat to the proper regions. Also, instead of using a source of heat to produce a temperature difference, it would be possible to use a cooling or refrigerating source, although such sources are commonly more expensive and bulky at the present time.

Although the light valves described are commonly used with visible light sources, with suitable suspensions it may be possible to control the passage of other types of electromagnetic radiation such as infrared and ultraviolet light. Also, instead of using continuous area electrodes within the active region of the cell, the electrodes may be formed in patterns so as to exhibit a desired display. Further, instead of allowing light to pass through the cell from front to rear, the rear surface may be made reflective so as to provide a mirror of variable reflectivity. It will be understood that the term light valve" applies to these various types of applications.

The invention has been described in connection with a number of embodiments thereof, and certain variations have been mentioned. it will be understood that other modifications are possible within the spirit and scope of the invention as defined in the claims.

We claim:

1. A light valve including a cell for containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid, said cell having spaced wall sections, and means for applying a respective electric or magnetic field, or both, to the fluid between said spaced wall sections to change the light transmission thereof, in which the improvement comprises conduit means for connecting substantially opposite edge portions of said cell, at least one of said conduit means and cell having a portion thereof extending in a direction having a substantial vertical component, and means for producing a thermal gradient in the fluid in said portion having a substantial vertical component to produce a flow of the fluid through said cell.

2. A light valve including a cell containing a fluid suspension of minute particles dispersed therein capable of being acted upon by an electric field to change the transmission of light through the suspension, said cell having wall sections spaced apart a distance which is small compared to the lateral dimensions of the sections to confine the fluid suspension therebetween to a layer, and area electrodes on opposite sides of said layer for producing an electric field through the layer to change the light transmission thereof, in which the improvement comprises conduit means for connecting substantially opposite edge portions of said cell, at least one of said conduit means and cell having a portion thereof extending in a direction having a substantial vertical component, and means for producing a thermal gradient in the fluid suspension in said portion having a substantial vertical component to produce a flow of the fluid suspension in said layer.

3. A light valve according to claim 2 in which the spacings of the walls of said cell at said opposite edge portions connected by said conduit means are substantially greater than the spacing of the wall sections in the region of said layer, thereby forming channels at opposite edges of said layer.

4. A light valve according to claim 3 in which a portion of said conduit means extends in a direction having a substantial vertical component, said means for producing a thermal gradient in the fluid suspension including heating means operatively associated with said portion of the conduit means for heating the fluid suspension therein.

5.' A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, and including heating means for heating a region of said cell extending laterally of the cell near the bottom thereof to produce said temperature gradient.

6. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, and including means for subjecting at least a portion of the fluid suspension in said layer and at least a portion of said conduit means to different ambient temperatures to thereby produce said temperature gradient.

7. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component, said channels are at upper and lower edges of said layer, and said conduit means is a conduit of heat-transmitting material positioned exteriorly of said cell and connecting said channels, and means for subjecting at least a portion of said cell and at least a portion of said conduit to different ambient temperatures to thereby produce said temperature gradient.

8. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, said conduit means comprising a barrier wall between the walls of said cell on one side of the region of said layer, said barrier wall being spaced from the corresponding side of the cell and spaced from the top and bottom of the cell to form a conduit between said upper and lower channels of the cell, the spacing of the walls of the cell between said barrier wall and the corresponding side of the cell being substantially greater than the spacing of the wall sections in the region of said layer.

9. A light valve according to claim 8 in which said means for producing a temperature gradient comprises electric heating means for heating the fluid suspension in said conduit.

10. A light valve according to claim 8 in which said means for producing a temperature gradient comprises electric heating means extending laterally of said cell below said area electrodes.

11. A light valve according to claim l0 in which said electric heating means comprises an electrical resistive coating on the inner surface of at least one of the walls of the cell below the area electrode on the respective wall and insulated therefrom.

12. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, said conduit means comprising a pair of barrier walls between the walls of said cell on respectively opposite sides of the region of said layer, said barrier walls being spaced from the respective sides of the cell and spaced from the top and bottom of the cell to form a pair of conduits between said upper and lower channels of the cell, the spacing of the walls of the cell between said barrier walls and the respective sides of the cell being substantially greater than the spacing of the wall sections in the region of said layer.

13. A light valve according to claim 12 in which said means for producing a temperature gradient comprises electric heating means for heating the fluid suspension in each of said conduits.

14. A light valve according to claim 3 in which said cell and at least a portion of said conduit means are at different vertical levels with a portion of the conduit means extending in a direction having a substantial vertical component, and said channels are at horizontally separated edges of said layer.

15. A light valve according to claim 14 in which said means for producing a thermal gradient includes heating means operatively associated with said portion of the conduit having a substantial vertical component.

16. A light valve according to claim 14 in which said cell is at a lower vertical level than a portion of said conduit means, and said means for producing a thermal gradient includes heating means operatively associated with one of said channels for heating the fluid suspension therein.

17. A light valve according to claim 16 in which said heating means comprises an electrical heating element in said one channel, and including a barrier wall in said one channel positioned between said heating element and the adjacent edge of said layer and spaced from the adjacent edge of the layer, one end of said barrier wall being spaced from the adjacent edge of the cell to allow flow of the fluid suspension thereby.

18. A light valve according to claim in which said cell is disposed generally horizontally and said flow of the fluid suspension in said layer is in a generally horizontal direction.

19. A light valve including a cell for containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid, said cell having spaced wall sections and means for applying a respective electric or magnetic field, or both, to the fluid between said spaced wall sections to change the light transmission thereof, in which the improvement comprises means for producing a thermal gradient in the fluid to produce a flow of the fluid through said cell when said fluid is being acted upon by said electric or magnetic field or both.

20. The light valve of claim 19 wherein the fluid is a fluid suspension.

21. A light valve including a cell for containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid, said cell having spaced wall sections and means for applying a respective electric or magnetic field, or both, to the fluid between said spaced wall sections to change the light transmission thereof, in which the improvement comprises means for producing a thermal gradient in the fluid to produce a flow of the fluid through said cell in a defined path to prevent significant agglomeration in the fluid.

22. The light valve of claim 21 wherein the fluid is a fluid suspension.

23. The method of preventing significant agglomeration in a fluid suspension in a light valve wherein the light valve includes a cell for containing the fluid suspension which is capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid suspension comprising the steps of producing a thermal gradient in the fluid suspension when said fluid suspenson is being acted upon by the electric or magnetic field or both and causing a flow of the fluid suspension through the cell by means of the thermal gradient.

24. The method of preventing significant agglomeration in a fluid suspension in a light valve wherein the light valve includes a cell for containing the fluid suspension which is capable of bring acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid suspension compris ing the steps of producing a thermal gradient in the fluid suspension and causing a flow of the fluid suspension through the cell in a defined path by means of the thermal gradient.

UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,788,729 DatedM Inventor(s) Lowell et al It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Page 1 of the patent, [75] line 2, "Forline" should read -Forlini-.

Column 1,

line 10, between "through" and "fluid insert Column 2, line 25, "setting" should read-settling- Column 3, line 7 between "produc ing" and "thermal" insert Signed and sealed this 9th day of July @1974.

(SEAL) Attest: v A A mccor M. GIBSON, JR. c. MARSHALL DANN Attesting Officer Commissioner of Patents FORM USCOMM-DC 60316-P69 v U.S. GDVIINIIINT PRINTING OFFICE 2 I," D J'334. 

1. A light valve including a cell for containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid, said cell having spaced wall sections, and means for applying a respective electric or magnetic field, or both, to the fluid between said spaced wall sections to change the light transmission thereof, in which the improvement comprises conduit means for connecting substantially opposite edge portions of said cell, at least one of said conduit means and cell having a portion thereof extending in a direction having a substantial vertical component, and means for producing a thermal gradient in the fluid in said portion having a substantial vertical component to produce a flow of the fluid through said cell.
 2. A light valve including a cell containing a fluid suspension of minute particles dispersed therein capable of being acted upon by an electric field to change the transmission of light through the suspension, said cell having wall sections spaced apart a distance which is small compared to the lateral dimensions of the sections to confine the fluid suspension therebetween to a layer, and area electrodes on opposite sides of said layer for producing an electric field through the layer to change the light transmission thereof, in which the improvement comprises conduit means for connecting substantially opposite edge portions of said cell, at least one of said conduit means and cell having a portion thereof extEnding in a direction having a substantial vertical component, and means for producing a thermal gradient in the fluid suspension in said portion having a substantial vertical component to produce a flow of the fluid suspension in said layer.
 3. A light valve according to claim 2 in which the spacings of the walls of said cell at said opposite edge portions connected by said conduit means are substantially greater than the spacing of the wall sections in the region of said layer, thereby forming channels at opposite edges of said layer.
 4. A light valve according to claim 3 in which a portion of said conduit means extends in a direction having a substantial vertical component, said means for producing a thermal gradient in the fluid suspension including heating means operatively associated with said portion of the conduit means for heating the fluid suspension therein.
 5. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, and including heating means for heating a region of said cell extending laterally of the cell near the bottom thereof to produce said temperature gradient.
 6. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, and including means for subjecting at least a portion of the fluid suspension in said layer and at least a portion of said conduit means to different ambient temperatures to thereby produce said temperature gradient.
 7. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component, said channels are at upper and lower edges of said layer, and said conduit means is a conduit of heat-transmitting material positioned exteriorly of said cell and connecting said channels, and means for subjecting at least a portion of said cell and at least a portion of said conduit to different ambient temperatures to thereby produce said temperature gradient.
 8. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, said conduit means comprising a barrier wall between the walls of said cell on one side of the region of said layer, said barrier wall being spaced from the corresponding side of the cell and spaced from the top and bottom of the cell to form a conduit between said upper and lower channels of the cell, the spacing of the walls of the cell between said barrier wall and the corresponding side of the cell being substantially greater than the spacing of the wall sections in the region of said layer.
 9. A light valve according to claim 8 in which said means for producing a temperature gradient comprises electric heating means for heating the fluid suspension in said conduit.
 10. A light valve according to claim 8 in which said means for producing a temperature gradient comprises electric heating means extending laterally of said cell below said area electrodes.
 11. A light valve according to claim 10 in which said electric heating means comprises an electrical resistive coating on the inner surface of at least one of the walls of the cell below the area electrode on the respective wall and insulated therefrom.
 12. A light valve according to claim 3 in which said cell extends in a direction having a substantial vertical component and said channels are at upper and lower edges of said layer, said conduit means comprising a pair of barrier walls between the walls of said cell on respectively opposite sides of the region of said layer, said barrier walls being spaced from the respective sides of the cell and spaced from the top and bottom of the cell to form a pair of conduits between said upper and lower channels of the cell, the spacing of the walls of the cell between said barrier walls and the respective sidEs of the cell being substantially greater than the spacing of the wall sections in the region of said layer.
 13. A light valve according to claim 12 in which said means for producing a temperature gradient comprises electric heating means for heating the fluid suspension in each of said conduits.
 14. A light valve according to claim 3 in which said cell and at least a portion of said conduit means are at different vertical levels with a portion of the conduit means extending in a direction having a substantial vertical component, and said channels are at horizontally separated edges of said layer.
 15. A light valve according to claim 14 in which said means for producing a thermal gradient includes heating means operatively associated with said portion of the conduit having a substantial vertical component.
 16. A light valve according to claim 14 in which said cell is at a lower vertical level than a portion of said conduit means, and said means for producing a thermal gradient includes heating means operatively associated with one of said channels for heating the fluid suspension therein.
 17. A light valve according to claim 16 in which said heating means comprises an electrical heating element in said one channel, and including a barrier wall in said one channel positioned between said heating element and the adjacent edge of said layer and spaced from the adjacent edge of the layer, one end of said barrier wall being spaced from the adjacent edge of the cell to allow flow of the fluid suspension thereby.
 18. A light valve according to claim 15 in which said cell is disposed generally horizontally and said flow of the fluid suspension in said layer is in a generally horizontal direction.
 19. A light valve including a cell for containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid, said cell having spaced wall sections and means for applying a respective electric or magnetic field, or both, to the fluid between said spaced wall sections to change the light transmission thereof, in which the improvement comprises means for producing a thermal gradient in the fluid to produce a flow of the fluid through said cell when said fluid is being acted upon by said electric or magnetic field or both.
 20. The light valve of claim 19 wherein the fluid is a fluid suspension.
 21. A light valve including a cell for containing a fluid capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid, said cell having spaced wall sections and means for applying a respective electric or magnetic field, or both, to the fluid between said spaced wall sections to change the light transmission thereof, in which the improvement comprises means for producing a thermal gradient in the fluid to produce a flow of the fluid through said cell in a defined path to prevent significant agglomeration in the fluid.
 22. The light valve of claim 21 wherein the fluid is a fluid suspension.
 23. The method of preventing significant agglomeration in a fluid suspension in a light valve wherein the light valve includes a cell for containing the fluid suspension which is capable of being acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid suspension comprising the steps of producing a thermal gradient in the fluid suspension when said fluid suspenson is being acted upon by the electric or magnetic field or both and causing a flow of the fluid suspension through the cell by means of the thermal gradient.
 24. The method of preventing significant agglomeration in a fluid suspension in a light valve wherein the light valve includes a cell for containing the fluid suspension which is capable of bring acted upon by an electric or magnetic field, or both, to change the transmission of light through the fluid suspension comprising the steps of producing a thermal gradient in the fluid suspEnsion and causing a flow of the fluid suspension through the cell in a defined path by means of the thermal gradient. 